At present, very large scale integrated circuits, currently known as VLSI circuits or chips, are substantially square small fine plates of semiconductor material, over 1 cm long on each side, provided with a great number--on the order of 300 to 400--of input/output terminals. In tape-automated bonding technology (TAB), each integrated circuit is mounted on a package, which comprises an insulating substrate including a window, with a bundle of leads overhanging the substrate and converging in the window. The input/output terminals of the integrated circuit are fixed by an operation known as inner lead bonding (ILB) in the window of the substrate, at the free ends of the leads. These terminals are in fact placed on the periphery of the integrated circuit, in a line or in zig-zag fashion. In current terminology, the package is called a TAB package, the bundle of leads a spider, the leads themselves the legs of the spider, and their inner ends the ILB ends. Ordinarily, the insulating substrate of the package is a flexible tape similar to motion-picture film, which is provided with successive windows for connection of the integrated circuits and lateral perforations for tape displacement and positioning.
It is presently possible for the spacing of the input/output terminals on a VLSI chip to be as close as less than 100 micrometers. Hence the leads for an integrated circuit of this kind are necessarily quite narrow and very close to one another. To rigidify them and assure that they are equidistant in the course of the various manipulations of the package and spider, the leads inside a window are generally supported by at least one insulating frame originating in the substrate of the package. A frame of this kind is currently available to support the ILB ends of the leads of a spider. In one case, the ILB operation is done near the frame, which rests on the integrated circuit. Otherwise, the frame is pierced with holes under the leads, for their ILB connection to the corresponding terminals of the integrated circuit. In both cases, the leads are separated from the corresponding terminals of the integrated circuit by a height equal to the thickness of the ILB frame. Consequently, the ILB connection is generally done with beads, which are currently made of a soldering material such as tin-lead alloy and of sufficient height to compensate for the thickness of the ILB frame, that are fixed or deposited on the input/output terminals of the integrated circuit. The problem presented by this frame in ILB soldering to the terminals of a VLSI chip has been solved up to now by a sophisticated method of manufacturing a TAB package.
A conventional TAB package is made by engraving a sheet of copper glued to a preformed substrate. This package accordingly comprises three layers: copper - adhesive - substrate. Ordinarily, the copper foil has a thickness of approximately 15 to 70 .mu.m, the glue is acrylic or epoxy spread over a thickness of approximately 25 .mu.m, and the substrate is a film of the motion-picture type, made of a flexible and electrically insulating material (such as kapton, Mylar(.RTM.), epoxy, etc.), the minimum thickness of which is about 50 82 m and currently is on the order of 100 .mu.m. All the openings in the substrate are made by mechanical cutting operations, such as punching. The ILB frame, formed by cutting a window in the substrate, is necessarily supported by the substrate, actually on arms disposed along the diagonals of the window. This package is not very expensive and its substrate is sufficiently thick to maintain considerable rigidity when it is large in size, that is, larger than 50 mm on a side. However, because its substrate is preformed by mechanical cutting, it it unusable for the very large scale density of spiders in current use. The ILB frame is made by cutting of the substrate and thus has the same thickness, hence 50 .mu.m at minimum, to obtain the requisite qualities of the package. It follows that each bead of solder necessary for fixing one lead of the spider to the corresponding terminal of the integrated circuit must have a thickness greater than the thickness of the package, and hence greater than 50 .mu.m. Such large beads cannot be made except on large terminals, and they require the piercing of large holes, which conflicts with the large scale integration desired for connecting the leads to an integrated circuit. Moreover, piercing of large holes in an ILB frame can be done on an industrial scale only by punching, which has the disadvantage of piercing holes of at least 600 to 800 .mu.m in diameter. It has proved impossible to adapt a standard TAB package to a spider for large scale integration.
The solution currently used for this problem resides in manufacturing a TAB package in two layers: copper and substrate. By this method, on a thin substrate approximately 20 .mu.m thick of insulating material--ordinarily, polyimide--a fine film of copper is deposited uniformly in a vacuum (generally by the intermediary of a bonding layer, for instance of chromium), which is then grown selectively by electrochemical means to form the TAB spider. The film of copper is removed except for the pattern of the spider, and then the substrate is engraved, which is made possible by its thinness. The engraving comprises for instance depositing on the substrate a film of silicon dioxide (SiO.sub.2), which is engraved with the aid of a film of photoresist, to serve as a mask in the oxygen plasma engraving of the polyimide substrate. The oxygen plasma attacks only the organic material, in other words the photoresist and the polyimide, but has the disadvantage of a relatively long penetration time, on the order of 1 .mu.m per minute. It will be understood that this method is unsuitable for industrial use with thick substrates. Contrarily, this method is quite suitable for thin substrates. In that case, the frames and particularly the ILB frame no longer need to be attached by arms to the rest of the substrate. In the case where the ILB frame must be pierced under the leads, the holes are advantageously made at the time the substrate is engraved with the oxygen plasma and can thus have a diameter of less than 100 .mu.m. The slight thickness of the ILB frame enables the ILB connection to be made by the intermediary of small beads of solder, which are adapted to the present very large scale of integration of these circuits, and the small perforations engraved in the frame are no longer obstacles to the large scale of integration desired. However, such a package costs about three times as much as a conventional package, and it proves to be difficult to use for large substrates, more than 70 mm on a side, for example, because its thinness means that the substrate is very highly flexible.